Bitumen still melting in Tassie heat

Tasmania's heatwave is melting the bitumen and setting up severe fire conditions.

Tasmania has been baking in above-average temperatures, even for summer, this week and while there was a slight reprieve on Thursday, it will heat up again on Friday.

A total fire ban has been declared for the entire state on Friday.

Tasmania Fire Service chief officer Mike Brown said the forecast for very hot, windy and dry conditions made it necessary to ban all fires in the open.

"The vegetation in most of the state has dried out and forecast conditions will cause any fires to develop quickly making control very difficult," he said.

The heat has been melting the bitumen with motorists urged to slow down and take extra care on some roads.

Shane Gregory, general manager of transport infrastructure services at the Department of Infrastructure, Energy and Resources, said bitumen hardens and softens depending on the temperature.

"This is a natural event which occurs not only in Tasmania and interstate, but across the globe," Mr Gregory said.

"Bitumen softening only becomes an issue on some roads where there was more bitumen sprayed.

"We're talking about fairly high temperatures on the road surface - 35 degrees can actually become 50 degrees on the road surface."

Affected areas include Constitution Hill, Spring Hill and the Lovely Banks area on the Midland Highway, Cherry Tree Hill on the Tasman Highway, and Lawitta on the Lyell Highway.

While the temperature was only in the early 20s in Hobart on Thursday on Friday the city will get to 38C.

It's remained in the 30s in Launceston with 36C forecast for Friday.

The central Highlands town of Ouse will hit 40C.